|Publication number||US8014676 B2|
|Application number||US 12/035,677|
|Publication date||Sep 6, 2011|
|Filing date||Feb 22, 2008|
|Priority date||Feb 22, 2008|
|Also published as||US20090214223|
|Publication number||035677, 12035677, US 8014676 B2, US 8014676B2, US-B2-8014676, US8014676 B2, US8014676B2|
|Inventors||Young-Kai Chen, Sanjay Patel, Mahmoud Rasras, Kun-Yii Tu|
|Original Assignee||Alcatel Lucent|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (22), Non-Patent Citations (16), Referenced by (1), Classifications (11), Legal Events (4)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This invention was made with Government support under Contract No. HR0011-05-C-0027 awarded by Defense Advanced Research Projects Agency (DARPA) under the EPIC (Electronic and Photonic Integrated Circuits) program. The Government has certain rights in this invention.
1. Field of the Invention
The present invention relates to microwave circuits and, more specifically, to microwave filters.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art and/or what is not in the prior art.
A band-stop or band-rejection filter is a filter that passes substantially unaltered most frequencies in its spectral range of operation, except for the frequencies in one or more specific spectral bands (stop bands), which are attenuated to a relatively low level. A band-stop filter performs a spectral function that is substantially opposite to that of a corresponding band-pass filter. A band-stop filter having a relatively narrow stop band is often referred to as a notch filter. Tunable band-stop filters find applications in communication systems, for example, at a receiver, to remove interference signals originating from co-located transmitters and/or from adjacent receive bands and, at a transmitter, to remove harmonic and spurious signals, e.g., caused by power-amplifier nonlinearities.
A microwave photonic filter is an optoelectronic (or electro-optic) circuit designed to perform functions that are analogous to those of a conventional microwave filter. As used in this specification, the term “microwave” designates electromagnetic signals having frequencies in the range from about 3 Hz to about 300 GHz. As such, this term covers radio-frequency (RF) signals and millimeter-wave signals in addition to what is traditionally referred to as microwave signals.
Microwave photonic filters have certain recognized advantages over conventional microwave filters. These advantages include, but are not limited to, a relatively low loss that is substantially independent of the signal frequency, relatively low sensitivity to electromagnetic interference (EMI), relatively low weight and small size, and amenability to spatial and spectral parallelism through the use of wavelength-division multiplexing (WDM) techniques. Implementing microwave photonic filters with standard silicon complementary-metal-oxide-semiconductor (CMOS) technology holds the promise of minimizing production costs, e.g., through monolithic integration of electronic and photonic functions and the use of the massive existing CMOS manufacturing infrastructure.
According to one embodiment, a microwave photonic band-stop (MPBS) filter uses an electrical input signal to drive an optical Mach-Zehnder modulator. A modulated optical carrier produced by the modulator is applied to an optical filter having at least two tunable spectral attenuation bands that are located substantially symmetrically on either side of the carrier frequency. The resulting filtered optical signal is applied to an optical-to-electrical (O/E) converter to produce an electrical output signal. Advantageously, the MPBS filter is capable of continuously tuning the spectral position of its microwave stop band between about 0 and about 20 GHz and is amenable to implementation in CMOS technology.
According to one embodiment, a microwave filter has an optical modulator adapted to modulate an optical signal having an optical carrier frequency to generate a modulated optical signal, said modulation being based on an electrical input signal. The microwave filter further has an optical filter having first and second spectral attenuation bands and adapted to filter said modulated optical signal to produce a filtered optical signal, wherein the optical carrier frequency is substantially centered between said first and second spectral attenuation bands. The microwave filter also has an optical-to-electrical (O/E) converter adapted to convert the filtered optical signal into an electrical output signal.
According to another embodiment, a method of processing electrical signals has the steps of: (A) modulating an optical signal having an optical carrier frequency to generate a modulated optical signal, said modulation being based on an electrical input signal; (B) filtering said modulated optical signal in an optical filter having first and second spectral attenuation bands to produce a filtered optical signal, wherein the optical carrier frequency is substantially centered between said first and second spectral attenuation bands; and (C) converting the filtered optical signal into an electrical output signal.
Other aspects, features, and benefits of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which:
MPBS filter 100 has a light source (e.g., a laser) 110 configured to generate a CW beam 112 and apply that beam to an optical modulator 130. A driver circuit 120 receives input signal 118 and transforms that signal into a drive signal 122 suitable for driving optical modulator 130. For example, driver circuit 120 might amplify and/or bias (i.e., shift the dc level of) input signal 118 to produce drive signal 122. Optical modulator 130 driven by drive signal 122 modulates CW beam 112 to produce a modulated optical signal 132. In one embodiment, optical modulator 130 is a double-sided LiNbO3 Mach-Zehnder modulator.
Modulated optical signal 132 is filtered by optical filter 140, the spectral characteristics of which are described in more detail below. A controller circuit 136 receives external control signal 134 and, based on that control signal, appropriately configures optical filter 140 for filtering signal 132. A resulting filtered signal 142 is amplified in an optional optical amplifier (OA) 150. An optical-to-electrical (O/E) converter 160 then converts an amplified optical signal 152 produced by OA 150 into electrical output signal 168. In the embodiment of
For MPBS filter 100 to produce a microwave stop band at about 5 GHz, optical filter 140 is configured to have a transmission spectrum 147 having two attenuation bands 148 a-b whose minima are (i) located substantially symmetrically on either side of carrier frequency fc and (ii) are spectrally separated from one another by about 10 GHz, with each minimum being spectrally separated from fc by about 5 GHz. This spectral configuration lines up attenuation bands 148 a-b with modulation sidebands 146 a-b, respectively, and causes optical filter 140 to substantially reject those modulation sidebands. Thus, the optical spectrum of filtered optical signal 142 has center band 145, second-order sidebands 146 c-d, etc., but does not have first-order sidebands 146 a-b.
The rejection of first-order sidebands 146 a-b causes the RF spectrum of electrical output signal 168 to have a dip at about 5 GHz. The shape (e.g., the depth and width) of the dip depends on the shapes of attenuation bands 148 a-b. For example, in
In one embodiment, optical filter 140 is continuously tunable so as to maintain attenuation bands 148 a-b at substantially symmetrical spectral positions with respect to carrier frequency fc, while being able to change the value of the spectral separation between the attenuation bands and the carrier frequency. This continuous tunability of optical filter 140 enables MPBS filter 100 to function as a continuously tunable microwave band-stop filter. More specifically, an increase in the spectral separation between attenuation bands 148 a-b will cause a corresponding shift of the microwave stop band of MPBS filter 100 to a higher frequency. Conversely, a decrease in the spectral separation between attenuation bands 148 a-b will cause a corresponding shift of the microwave stop band to a lower frequency.
Each of optical couplers 212 a-b is tunable and is configured to control the optical coupling strength between MZI arm 204 a and the corresponding one of resonators 210 a-b. In one embodiment, optical coupler 212 is a thermo-optic coupler whose coupling strength depends on the temperature. Accordingly, TOF 240 is adapted to tune optical couplers 212 a-b by changing their respective temperatures. Several thermo-optic coupler designs, each suitable for implementing optical coupler 212, are disclosed, e.g., in U.S. patent application Ser. No. 11/869,205, which is incorporated herein by reference in its entirety. In various other embodiments, methods, such as carrier injection, carrier depletion, stress, photorefractive effects, or other techniques that enable controllable change of the effective refractive index of waveguide material(s), can be used as a physical principle of operation of optical coupler 212.
A suitable phase shifter that can be used to implement each of phase shifters 208 and 218 is disclosed, e.g., in U.S. Patent Application Publication No. 2006/0045522, which is incorporated herein by reference in its entirety. Phase shifters 208 serve to adjust the relative phase difference between the optical sub-beams in MZI arms 204 a-b, e.g., when at least one of optical couplers 212 a-b has been tuned. Phase shifter 218 serves to control the effective optical length of the respective resonator 210, which length depends on the optical phase accrued by the optical signal in the phase shifter.
The various loss curves shown in
The coupling strength set by optical coupler 212 controls the partition of light between the direct propagation path and the loop “detour” path through optical resonator 210 and, therefore, determines the extent of light extinction due to the interference between the light entering the optical coupler from MZI arm 204 a and the light entering the optical coupler from within the optical resonator. The bandwidths and the nulling depths of the corresponding spectral attenuation bands are determined by the coupling strengths set by optical couplers 212 a-b and the amount of nonlinear phase introduced into MZI arm 204 a by resonators 210 a-b. The nonlinear phase produces a group-delay difference between MZI arms 204 a-b, the amount of which is controlled by the coupling strengths. In general, a desired amount of optical group delay in MZI arm 204 a can be obtained by tuning thermo-optic couplers 212 a-b to change the coupling strengths between the MZI arm and optical resonators 210 a-b, respectively. A good quantitative description of the effect of coupling strength on resonator-induced optical group delay can be found, e.g., in an article by G. Lenz, et al., “Optical Delay Lines Based on Optical Filters,” IEEE Journal of Quantum Electronics, 2001, v. 37, No. 4, pp. 525-532, which is incorporated herein by reference in its entirety. In a representative embodiment, optical coupler 212 can be used to tune the magnitude (nulling depth) of the corresponding attenuation band between about 0 and 30 dB.
In one configuration, optical couplers 412 a-b are configured similar to optical couplers 212 a-b, respectively. In contrast, optical couplers 412 c-d are configured to substantially decouple optical resonators 410 c-d from MZI arm 404 a. As a result, TOF 440 functions and can be operated similar to TOF 240.
In another configuration, optical resonators 410 a-b are configured to produce a first pair of spectral attenuation bands analogous to attenuation bands 148 a-b (see
Various embodiments of tunable MPBS filters of the invention advantageously provide some or all of the following benefits. An MPBS filter of the invention can be implemented in a single integrated CMOS circuit, which makes the filter amenable to large-volume and low-cost production. An added benefit of the CMOS compatibility is that the relatively high refraction-index contrast of silicon/silicon oxide optical waveguides enables relatively high packing densities of circuit components, which leads to advantageously compact devices. In addition, the relatively high refraction-index contrast of silicon/silicon oxide waveguides can be used to create and utilize optical resonators with relatively large FSRs. In a representative CMOS implementation, an MPBS filter of the invention is capable of providing a notch-like stop band having a 3-dB bandwidth as narrow as about 0.1 GHz with the spectral position of that stop band being continuously tunable from about 0 GHz to about 20 GHz. The MPBS filter is substantially immune to RF EMI and has a lower overall complexity than a functionally comparable conventional microwave filter. When implemented in CMOS silicon, the MPBS filter is generally capable of using optical carrier frequencies from the C-band (1525-1565 nm), thereby taking advantage of the availability of a large variety of coherent light sources developed for that spectral range in optical communications applications.
While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Although a representative MPBS filter of the invention has been described with respect to possible implementation as a single integrated circuit, it can also be implemented as a multi-chip module, a single card, or a multi-card circuit pack. MPBS filters of the invention can be implemented in any suitable technology different from CMOS. Various modifications of the described embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the principle and scope of the invention as expressed in the following claims.
Unless explicitly stated otherwise, each numerical value and range should be interpreted as being approximate as if the word “about” or “approximately” preceded the value of the value or range.
It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the scope of the invention as expressed in the following claims.
It should be understood that the steps of the exemplary methods set forth herein are not necessarily required to be performed in the order described, and the order of the steps of such methods should be understood to be merely exemplary. Likewise, additional steps may be included in such methods, and certain steps may be omitted or combined, in methods consistent with various embodiments of the present invention.
Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.”
Also for purposes of this description, the terms “couple,” “coupling,” “coupled,” “connect,” “connecting,” or “connected” refer to any manner known in the art or later developed in which energy is allowed to be transferred between two or more elements, and the interposition of one or more additional elements is contemplated, although not required. Conversely, the terms “directly coupled,” “directly connected,” etc., imply the absence of such additional elements.
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|US9201287||Apr 28, 2015||Dec 1, 2015||Opel Solar, Inc.||Photonic analog-to-digital converter|
|U.S. Classification||398/85, 398/198, 398/183, 398/185, 398/192, 398/82|
|International Classification||H04B10/04, H04J14/02|
|Cooperative Classification||H04B2210/006, H04B10/00|
|Apr 29, 2008||AS||Assignment|
Owner name: LUCENT TECHNOLOGIES INC., NEW JERSEY
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Owner name: LUCENT TECHNOLOGIES INC., NEW JERSEY
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